To improve the performance of an internal combustion engine, a supercharger (also referred to as a turbocharger) that drives with an exhaust gas of an internal combustion engine, compresses intake air, and threes compressed intake air to the internal combustion engine is used. An electric supercharger that uses an electric motor as a drive source instead of a turbine has become common. In the electric supercharger, the electric motor is disposed at a shaft of the electric supercharger. The electric motor rotationally drives a compressor so as to improve acceleration responsiveness of the electric supercharger. The rotation shaft of the electric supercharger is provided with a rotor made of a permanent magnet, an iron core, or the like.
Even if the rotational balance of each structural member that composes a rotor of a compressor wheel or the like of the electric supercharger is accurately adjusted, the rotational unbalance of the whole assembly of the individual structural members may be lost due to an assembling error or the like. If the unbalanced rotor is rotated at a high speed, the rotor may vibrate or break. Thus, before the electric supercharger is shipped, the rotor is rotated so as to contactlessly accurately measure the rotational unbalance of the rotor. A circumferential portion of the rotor that causes the rotational balance of the rotor to be lost is machined by shaving based on the measured result so as to correct the rotational unbalance of the rotor. Patent Document 1 discloses a related art reference that contactlessly measures a position and an amount of a portion where a rotational unbalance occurs in a rotor. In the method disclosed in Patent Document 1, a magnetized nut is mounted on a part of the rotor. Reference direction and position are detected by the magnetized nut. In addition, rotational unbalance portions and rotational unbalance amounts of the rotor are measured based on the reference direction and position.